Mechanisms insertable into a backrest to adjust shape and provide better lumbar support are known. Examples are found in U.S. Pat. Nos. 5,050,930 and 5,397,164 to Schuster et al.
Prior art mechanisms comprise a shaping element, commonly referred to as a "lumbar basket", which is mounted for displacement along a guide track. The lumbar basket may have various configurations. A basic construction involves a pair of brackets displaceable along an axis of the guide track, resilient axial ribs joining the brackets, and resilient transverse ribs fixed centrally to the axial ribs with free ends extending laterally to either side of the axial ribs to provide a cushioning effect. Various mechanisms can be used to draw the brackets together in order to flex the lumbar basket from a relative flat rest state to various bowed states. Various mechanisms can also be used to displace the lumbar basket axially along the track. Thus, the curvature of the lumbar basket and its position within a backrest can be adjusted to provide greater comfort.
The basic lumbar basket described above has a flexed profile which is essentially a segment of a circle, and consequently does not conform adequately to the curvature of a user's spine. A prior art approach to altering the flexed profile involves fixing a partial central rib to an upper bracket and an upper set of the transverse ribs, making the upper end of the basket more rigid. This induces greater flexing of the basket proximate to the lower bracket, providing greater comfort for many users. There are, however, shortcomings to such an approach. Making the partial rib and then fastening it to multiple components of the basic lumbar basket contributes to cost. There is also little freedom to specify the profile ultimately presented by the basket. Various alternatives can be envisaged to produce baskets that flex to various profiles; however, the basic prior art lumbar basket is simple, and it would be desirable to avoid introducing components and manufacturing steps.